Want a fun, relatively easy project to build? Ross Hershberger's Monobox Amplifier build on the Make Projects website is a great choice. This is an overview of my experience building it using an old army ammunition can, rather than a cigar box as Ross did. It came out really great, and it's something I continue to enjoy as it makes a great companion to my iPod.
This project was featured on the Make Blog: MonoBox
Mods: Same Circuit, Two Builds
I had intended to use a cigar box as I liked the look, but I didn't have any, and the ones I looked at on Ebay were too small. The project calls for an enclosure that has .125 to .25 cubic foot of space, and for a cigar box that seems to be rather large. Remembering that I had some old ammo cans laying around, I measure them and found that the .50 caliber ammo can has right around .25 cubit foot of volume, so it seemed liked a good candidate for the project. It's rough looking, but I liked that as I intended for it to have a vintage feel. Best of all, I already own it, so it's free. .50 cal boxes are relatively common and inexpensive, so if you're interested in doing this, you shouldn't have a problem getting one. As of the time I'm writing this, ArmySurplusWarehouse.com is selling them for less than $15, and reviews indicates they are in excellent condition.
The project is sponsored by Radio
Shack, so links are provided to locate all the parts on their
website. I found several of the parts at my local store. Several other
parts I purchased from Parts
Express, where I also got the grill cloth and Polyfill. For the
film capacitors, I went to Ebay because I didn't have any big orders I
needed to place, and I bought some vintage NOS caps from a seller that
specializes in parts for guitars. I'm not one to believe that using some
vintage caps will give my amp sweet vintage tone, but they were cheap, the
seller was in the same state (so I got them quicker) and what the heck,
they look cool.
The speaker is an Aura NS525-255-8A purchased from Madisound for less than $4. I chose it because it's cheap, had good sensitivity (89db) and a resonant frequency that meets the suggestions from the Monobox project page. I noticed after buying it that it's actually one of the speakers pictured on the project page, which leads me to wonder if Hershberger had tested it for use with this project.
Ross built a gasket for his speaker out of cork. The Aura has a gasket
on it already, but I thought it might not hurt to make one anyways and be
sure there was enough clearance. I bought some Quartet brand cork tiles
from Walmart. For about $6, you get four 12"x12" titles. They're about
3/16" thick. Using a compass ensures a perfect circle, but pencil will
not mark on the tile. I resolved this by attaching a black paint marker
to a compass using zip ties, as seen in the above photo. I also threw in
a photo of my compass set since I think it's somewhat interesting. I
don't know much about it other than it's made by Eugene Dietzgen Company
and it's been around awhile. It belonged to an uncle of mine and I
inherited it. CAD software is great, but it's still nice to break out the
compasses for a drawing from time to time.
The circuit is fairly easy to build, but I did run into a few difficulties. In a couple places, it's necessary to insert two wires/leads into one hold on the PCB. The vintage caps I used had thicker leads than the ones listed from Radioshack, so inserting the .010µF and .022µF capacitors together that make capacitor C1 was not possible without modification. It seems like a cheap trick, and it's probably not considered a good idea, but I ground down the leads using a Dremel to make them fit. I had to do the same thing in another spot where a lead and a jumper went into the same hold. I assume if you use the Radioshack parts listed on the project page, this won't be an issue.
For the initial test, I connected the circuit to a bench power supply (Parts Express #120-536, quite good for the price) set to 9v DC and I connected the speaker wires to a small 3" speaker. I was happy to find that it worked on the first try! With the electronics done, now I just needed to build the enclosure.
The enclosure needed a 5.25" hole cut out for the speaker and several holes drilled for screws and power and signal jacks. The hole for the speaker was the most work, but it came out okay. I took my time, running two passes with a cutting wheel mounted on a Dremel (actually, 4 cutting wheels, they wore out fast.) After cutting I cleaned up the edge with the Dremel using a grinding wheel and then a sandpaper wheel. It came out better than expected.
The circuit board is mounted at the bottom of the can upside-down using
some 1" nylon washers to hold it up. A nice thing about the ammo can is
that there is a large indentation in the bottom to make the cans stackable
(so the handles have room). This is nice because there's enough room so
that any screws installed with be "sunk in" and won't interfere with the
can sitting level on a table or other flat surface.
The speaker is mounted with the cork gasket and some nylon washers to ensure it's secure. I also mounted a toggle switch which allows selection between the external power and battery. I installed in inside the case so it won't be damaged as I see this as something that will get hauled around quite a bit. A 9v battery holder is mounted to the lid, also on the inside.
With that done, I filled the can with Polyfill and it was looking good!
It has a very vintage feel to it, due to the beat-up ammo can and the very
classic speaker cloth (Parts Express #261-810). The sound is good,
certainly not audiophile quality, but much better than I expected. It's
not meant for critical listening, it's meant to be a a fun, portable
speaker with lots of DIY charm that sounds good. In that regard, it
exceeds expectations.
With all of that done, the only problem left to tackle is rattling from metal hitting metal. This is a risk when using a metal enclosure, but I was able to fix it. The rattle was occurring in two places - between the lid overhang and the bottom portion of the can, and between the lid and top carrying handle. To stop the rattle, I cut up some scrap sweeper belts that are about .06" to .07" thick. I bent the lid a bit to make room, and they fit. They're glued in place with E-6000. If you try this, keep in mind that belts are generally smooth on both the ID and OD - they need to have the surface roughed up to get them to adhere to the glue. In my case, I was lucky as I had obtained some ground belts from work.
Here is a video of the finished product:
